1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, and a facsimile machine, and more particularly to an improvement of its fixing device.
2. Description of the Related Art
As a fixing device of an electrophotographic image forming apparatus such as a copying machine or a printer, there is hitherto well known a fixing device for carrying out fixing by a pair of roll-shaped or endless belt-shaped fixing rotating bodies which are brought into press contact with each other. That is, an unfixed toner image formed on a recording material by an electrophotographic process is made to pass through a nip portion of the pair of fixing rotating bodies, so that the toner image on the recording material is fixed on the recording material by heat and pressure from the fixing rotating bodies at that time, and a permanent image is obtained.
In such a fixing device, an unfixed toner is put in a melted state when passing through the nip portion of the pair of fixing rotating bodies, so that the toner is apt to adhere to the fixing rotating body with which the toner comes in contact. Thus, there have been problems that the recording material itself is not separated from the fixing rotating body to cause a jam, the toner adhered to the fixing rotating body stains a subsequent recording material, and so on. Accordingly, such a method has been generally adopted that a release agent such as silicone oil is applied to the surface of the fixing rotating body to improve a release property between the toner and the fixing rotating body.
Here, in order to make separation without fail, it is necessary that the release agent is uniformly applied to the surface of the fixing rotating body. On the other hand, if the release agent is excessively applied, there occur problems of poor image quality, especially poor reproducibility of color of a full color image, poor light transparency of an OHP sheet, and the like. Then, for the purpose of uniformly applying the smallest possible amount of release agent to the surface of the fixing rotating body for a long period of time, various proposals have been hitherto made.
For example, Japanese Patent Unexamined Publication No. Hei. 3-267979 discloses a technique for impregnating a cleaning web of a fixing rotating body with a release agent, Japanese Patent Unexamined Publication No. Hei. 8-123235 discloses a technique of using a mixture of release agents with different coefficients of viscosity, Japanese Patent Unexamined Publication No. Hei. 9-9681, No. Hei. 9-90737, and No. Hei. 5-181384 disclose a technique of doping an outermost surface layer of a fixing rotating body with a release agent directly or with a capsuled release agent, and Japanese Patent Unexamined Publication No. Hei. 4-168466 discloses a technique of using wax as a release agent.
However, higher quality has been required for an electrophotographic image, and it can not be necessarily said that offset can be prevented without fail and a sufficiently high quality image can be secured by these techniques. Particularly, in the case where a fixing rotating body deteriorated by use with a lapse of time or the like is used, there is a fear that if the amount of the release agent is small, the offset occurs immediately, so that it becomes necessary to replace the fixing rotating body.